A 45-year-old man with WPW syndrome and AF develops a rapid, irregular wide-complex tachycardia (rate 260 bpm) with a delta wave visible during slower beats. He is haemodynamically stable. Which pharmacological intervention is contraindicated and why?

• A Procainamide — increases bypass tract refractoriness, slowing conduction through accessory pathway
• B Verapamil — blocks AV node, increases anterograde conduction via accessory pathway, risking VF ✓
• C IV amiodarone — acceptable as it prolongs accessory pathway refractory period
• D Flecainide — blocks fast sodium channels in the accessory pathway

Explanation

In WPW-associated AF (pre-excited AF) with rapid conduction via the accessory pathway, AV nodal blocking drugs (verapamil, diltiazem, adenosine, digoxin) are contraindicated because they increase the ventricular response by removing the AV node's rate-limiting function, allowing all atrial impulses to conduct rapidly through the accessory pathway, which can degenerate into ventricular fibrillation. Procainamide (or flecainide) is preferred as they prolong accessory pathway refractoriness. Cardioversion is definitive therapy if haemodynamically unstable.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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